The outermost part of Gram-negative bacteria is the lipopolysaccharide (i.e. LPS). This molecule constitutes a permeability barrier that protects bacteria from a variety of noxious agents, and it is recognized by the host innate immune system of different animals and targeted by several natural antimicrobial peptides. Many Gram-negative pathogens have evolved mechanisms to modify their LPS in ways that decrease recognition by the host and increase resistance to antimicrobial peptides and certain toxic metals. In Salmonella enterica, which is the etiologic agent of typhoid fever and gastroenteritis, many of these LPS modifications are observed only under conditions that activate the PmrA/PmrB regulatory system. This proposal describes experiments aimed at understanding how the activity of the PmrA/PmrB system is dynamically controlled in response to internal and external inputs. Our studies will focus on a novel gene that encodes both a small peptide and small RNA, and on a set of Salmonella-specific genes that are required for survival inside macrophages. In addition, we will analyze the distinct properties that the PmrD protein plays in Salmonella, where it functions to activate the PmrA protein, and in the related species Escherichia coli, where it lacks this ability. We will examine whether the ability of commensal E. coli to colonize the mouse intestine is compromised when E. coli carries out the PmrA-controlled LPS modifications under inducing conditions that promote such activation in Salmonella. An accomplishment of these goals will uncover how the PmrA/PmrB system - a major regulator of LPS modifications in enteric bacteria - controls the remodeling of its cell surface in ways that protect it from antimicrobial insult. PUBLIC HEALTH RELEVANCE: Bacteria often modify their cell surface in ways that alter their resistance to antimicrobial agents. The proposed research will uncover the mechanisms that result in differential control of cell surface determinants in the pathogen Salmonella enterica and in the normal member of the human flora Escherichia coli.